I found a bit more on the letter I (long /i/, short /ɪ/, consonant /y/ before vowels = long feet, short tittle, consonant yes).
In compounds of iaciō, where the post-i ‘a’ is transformed into an ‘i’ [con-iaciō -> con-iiciō -> con-iciō], although the second i is no longer written within Latin script, it was apparently still pronounced within Latin speech. Thus, is is /kɔnyɪkyo/ and not /kɔnɪkyo/ (for those not-versed in IPA – it’s ‘con-yicki-o’ not ‘con-icki-o’).
This has been deduced by analysis of verse poetry, since the ‘o’ in con-iiciō would be scanned long if the first ‘i’ operates as a consonant, but scanned short if the speaker were merely voicing con-iciō.
The Essential AG: 6d, 11e